"GO" commonly means "FIVE" in Japanese. However, in the case of Gosanke,
it does NOT mean "FIVE", it means honorable, great....
When Japan was in its isolation period, the leader of Japan has three sons. The three sons grew up and was each given a top house. The Gosanke was the original term used to describe these three top houses.
Go = honorable, great
San = three
Ke = house
As time passed, Gosanke became a more generic term for "Top Three" or "Big Three". It can apply to anything like Singers, Dancers, and Koi fish as well.
Regards,
--Dinh